The invention relates to a developing solvent for layers which are crosslinkable by photopolymerization and contain a binder based on an elastomeric polymer, a photopolymerizable monomer compatible therewith and a photoinitiator.
It has long been known to produce relief forms, in particular flexographic printing plates, by means of photopolymerization-crosslinkable layers which are exposed imagewise and thereafter developed and washed out.
For the production of these forms, the photopolymer layer is exposed imagewise to actinic light; a relief can then be formed by washing off the non-exposed and thus non-crosslinked portions of the layer, using a developing solvent. The developing solvent should dissolve the non-crosslinked portions of the layer as quickly as possible, and the solvent must be removable with greatest possible ease from the crosslinked portions of the layer, so that the plate is rapidly dried. For this reason, low-boiling developing solvents presently are generally used.
Examples of preferred solvents which are described in DE-A 22 15 090 include methyl ethyl ketone, benzene, toluene, xylene, carbon tetrachloride, trichloroethane, trichlcoroethylene, methyl chloroform and tetrachloroethane, and mixtures of these.
It is a disadvantage of the above-indicated chlorinated hydrocarbons that they are toxic and give rise to disposal problems. If chlorinated hydrocarbons are used as developing solvents, the portions of the layer which are to be removed swell very strongly while still on the plate and as the concentration of dissolved components of the non-crosslinked layer portions increases in the developing solvent, viscosity of the solvent rises sharply. As a result, the capacity of the developing solvent is very limited and the solvent is rendered useless, already at a solids content of 5%. A further consequence of the strong increase in viscosity is that plates develop very slowly even at less than 5% solids content. In addition, development of plates which contain nitrile rubbers as the elastomers is very slow.
Of the non-chlorinated hydrocarbons specified in DE-A 22 15 090 benzene, toluene and xylene are mentioned. These solvents have the disadvantage of being easily flammable, moreover, they lead to severe swelling of the layer and, consequently, to slow developing and drying of the plate.
Additionally, due to their low flash-point, these solvents cannot be used in developing apparatuses which are not explosion-proof.
DE-A 36 00 116 describes relatively highly boiling developing solvents which contain hydrocarbons, alcohols or ketones which are branched, or cyclic with one or three olefinic double bonds, or saturated, or cycloaliphatic with one to three olefinic double bonds. Monoterpenes are mentioned in particular. Limonene which is used in the example may have good developing characteristics, but is sensitive to oxygen and irritates the skin, properties which should not be present in a developing solvent. It is also a disadvantage of limonene that it permits only very slow development and is difficult to remove from the layer. Furthermore, plates containing nitrile rubbers cannot be developed within reasonable time.